1. Field of the Invention
The present invention generally relates to the flow of bypass coolant through nuclear reactor internals and more particularly to the conversion of a downward flow to an upward flow to reduce the pressure difference between the bypass and core regions.
2. General Background
Many existing nuclear reactor internals were originally designed to be cooled by coolant bypass flow entering through the side of the core barrel adjacent the top and then flowing downwardly through holes in the former plates between the core barrel and the baffle plates. The coolant then joined the main coolant flow which flowed upward through the core region. However, the two flow directions resulted in a large pressure difference between the bypass region and the core region. This pressure differential of as much as 20 p.s.i. caused flow jetting at the gaps between adjacent baffle plates. This flow jetting resulted in damage to fuel pins which led to fuel pin failures. Attempts to reduce the pressure differential to a level which does not result in damage to fuel pins have traditionally plugged the horizontal core barrel flow holes and added vertical flow holes to the upper former plates. This form of upflow conversion results in the pressure differential being reduced from approximately 20 p.s.i. to approximately 3-4 p.s.i. However, even this reduced pressure differential still has sufficient driving pressure to cause jetting which leads to fuel pin failures for certain gap sizes between adjacent baffle plates. It can be seen that an upflow conversion method is needed which will reduce the pressure differential between the bypass and core regions to a level which does not result in jetting of coolant with subsequent fuel pin failures.